The preferred embodiments of the present invention relate to a process of two-photon induced photopolymerization for the formation of microstructures. More particularly, the preferred embodiments of the present invention relate to holographic two-photon induced photopolymerization (xe2x80x9cH-TPIPxe2x80x9d) in the construction of reflection and transmission holograms.
Molecular excitation via the simultaneous absorption of two photons can lead to improved three-dimensional (xe2x80x9c3Dxe2x80x9d) control of photochemical or photophysical processes due to the quadratic dependence of the absorption probability on the incident radiation intensity. This has lead to the development of improved 3D fluorescence.
Recently, the ability to fabricate 3D optical storage devices and ornate 3D microstructures has been demonstrated using two-photon induced photopolymerization (TPIP). This method requires sequential scanning of a series of extremely short (100-150 fs), high-peak-power (xe2x89xa7100""s xcexcW) laser pulses in a tightly focused single-beam geometry to cross the TPIP initiation threshold.
Others have sought to intersect two or more separate beams within a 3D photoactive material to induce simultaneous two-photon absorption exclusively within their respective intersection volumes. More recently, still others have sought to create three dimensional optical storage devices using a single highly focused beam to induce simultaneous two-photon excitation within one spatial region. Using this technique, arrays of photo-induced structures are then made by serially scanning the focused beam within a three dimensional photoactive material.
The microfabrication of microelectromechanical systems (MEMS) for biotechnology applications (bioMEMS) is a rapidly growing field. High-throughput DNA analysis is of central importance due to the many applications including, for example, chemical/biological weapons (CBW) defense. Bioagent detection, e.g., anthrax spores, can be accomplished quickly and specifically using high-throughput DNA analysis based on standard techniques like polymerase chain reaction (PCR). For industry, high-throughput DNA analysis has implications for the Human Genome Project.
The industrial challenge of completing the Human Genome Project is based on miniaturization technology which allows for more DNA to be analyzed in dramatically shorter time frames (minutes versus hours).
To this end, there is currently a push towards a lab-on-a-chip which makes use of conventional photolithography and laser induced fluorescence imaging to create a microfluidic plate containing site specific functionality. Microfluidic chips have the potential to synthesize thousands of individual molecules in microchannels in minutes, instead of the hours or days traditionally needed. The target species lab-on-a-chip microfluidic experiments are DNA, pathogens, toxins, cell-specific, and protein-specific. Various exemplary applications for utilization of lab-on-a-chip microfluidic experiments are environmental monitoring, biological warfare detection, cell sorting, protein separation, medical diagnostic, filtration, etc. The basic premise is to fabricate microfluidic channels, which have some form of functionality associated with individual pathways or micro-reservoirs. This functionality can take the form of enzymes, antibodies, DNA binding proteins, catalytic agents, etc. The injection of a sample, usually on the order of picoliters, flows through the microfluidic plate, causing reactions in various channels or test sites. The unreacted or unbound material is rinsed away. Computer readout of the various reactions is accomplished by tagging the individual reaction sites with a fluorescence dye. A laser is scanned through the plate and fluorescence imaging is used to map the reacted sites.
The manufacturing of these microfluidic chips relies primarily on conventional photolithography. Functionality is imparted after the fact by infusing the chip with some sort of polymer having the incorporated reactive species within it. Various masks are used to isolate different reactive species to specific areas of the chip. Thus, construction of the chips can take a good deal of time. Also, integration of the microlabs into the outside world is a significant challenge, while laser scanning is fine for laboratory work, portable or smaller versions would require something better. Attempts have been made to fiber couple these systems to a portable computer, but this technique has met with little success. A possible solution to that particular problem is being addressed through the conventional multiphoton laser ablation and micro-machining technologies. Industry leaders such as Clark MXR, SpectraPhysics and university collaborators are using high intensity lasers to ablate a channel and to form a waveguide perpendicular to the channel. While this would allow direct fiber optic coupling to the channel, the process is still a serial process, has low resolution, and does not give the elegant finesse required for more complex multidimensional forms.
The ability to reproduce naturally occurring micro and nano-structures is also highly sought after. One major roadblock to being able to image and reproduce these bio-structures is the fact that many conventional imaging technologies utilize ultra-violet wavelengths. Ultra-violet wavelengths are destructive to biological tissue.
Recent advances in conducting polymers to be used as an inexpensive supplement to conventional semiconductor, metallic, and ceramic components, have outpaced the corresponding fabrication techniques, which still rely upon one-photon polymerization and photolithographic technologies. Consequently, fabrication techniques for electronic circuits which utilize these conducting polymers are struggling to keep up with improvements to the conducting materials.
Also, the demand for low cost, high capacity data storage is increasing in both the military and the commercial sector. In the commercial sector, demand is primarily driven by the increasing digital format in the expanding computer, entertainment, and medical diagnostic areas. The military demand is primarily dictated by the data storage for hyperspectral data/image gathering and simulations. In short, increased optical data storage capacity and speed will be driven by going into the third dimension and by increasing multiplexing (spatial or frequency).
By taking advantage of all three dimensions instead of a planar storage configuration, data storage capacities of 1-10 terabits/cm3 are theoretically possible in the visible spectrum (P. J. Van Heerden, Appl. Opt., 2, 393-400 (1963)). Viewed mostly as a secondary or tertiary (archival) memory device, optical data storage is characterized by huge amounts of low cost per megabyte storage capacity with moderate access times (longer than 10 ms). Despite the low cost per megabyte capacity, storage could easily represent the single most expensive element in such large-scale operations as super computing. Several technologies have been considered for 3D data storage including layered 3-D optical storage, holographic data storage, persistent spectral hole burning, and near-field optical storage.
Because persistent spectral hole burning requires cryogenic temperatures and near-field optical storage has stringent requirements on the placement of the optical probe, their near-term commercialization is questionable. However, layered 3-D optical storage is a logical extension of current optical disk technology in which the information is simply stacked. The volume resolution (i.e., Mb/cm3) of these techniques is based on the precision of the read/write optics, the wavelength, and the resolution of photoactive media.
Further, the infrastructure of the telecommunication, display, and future computing industries rely heavily on the ability to switch information rapidly and with high contrast between two or more different states. The recent development of switchable diffractive elements using, e.g., polymer-dispersed liquid crystals (PDLC), is of interest in this context. These elements are currently formed using one-photon holographic photopolymerization of a reactive monomer mixed with inert liquid crystal material. The holographic overlap of two coherent beams results in a spatially modulated intensity pattern caused by the constructive and destructive interference of the incoming light. Since the rate of polymerization is related to the local intensity, spatially periodic polymerization occurs and subsequent phase separation results in the formation of planes of small LC domains separated by dense polymer regions.
Advantages of using holography over lithography and other surface-mediated structures include (a) the formation of small periods ( less than 1 micron) with high aspect ratios ( greater than 50) (e.g. .Bragg gratings), (b) single-step processing, and (c) writing various complex structures, including reflective gratings. The morphology formed is related to the interference pattern established by the holographic overlap and subsequent polymerization response of the monomer. The balance between the polymerization propagation and phase separation controls the final morphology of the two-phase composite: Methods to skew this balance and thus the final morphology across the Bragg period are currently being investigated.
The preferred embodiments of the present invention relate to both a process and photoactive media for holographic recording and micro/nanofabrication of optical and bio-optical structures. In particular, the preferred embodiments are concerned with the simultaneous absorption of two-photons by the photoactive media to induce a photochemical change in regions of constructive interference within a holographic pattern. According to embodiments of the present invention, the photochemical process of polymerization resulting from the simultaneous absorption of two-photons may be used for the microfabrication of micro and nanoscaled features, holographic data storage, and the formation of switchable diffraction gratings.
In contrast to the conventional techniques described above, the preferred embodiments of the present invention offer the ability to initiate arrays of photoactive sites in parallel by interfering two relatively unfocused beams while maintaining the potential increase in spatial resolution that is inherent to this nonlinear process. Because the two-photon process can be initiated with long wavelength radiation that has higher transmission for most materials, it is also possible to write 3D holographic structures in relatively thick photoactive materials.
Two-photon holography in the preferred embodiments involves the interference of two or more coherent beams to create regions of high and low intensity in which the two-photon excitation can be made to occur in the high intensity regions. The excitation by the simultaneous absorption of two-photons has several advantages over one-photon excitation in that the occurrence of absorption varies with the square of the intensity as opposed to linearly as known for one-photon excitation. Therefore, an intensity threshold should be surpassed before the process will occur. This can decrease the excitation volume to a subset of the irradiated area and can offer the potential for an increase in spatial resolution of the product of the excitation. The ability to induce two-photon excitation holographically allows for multiple spatial locations of excitation and the potential for rapid feature formation.
H-TPIP may be used to fabricate structures ranging in complexity from, for example, a spiral to a photonic bandgap. Conventionally, a fixed, confocal beam geometry was used to initiate two-photon polymerization while the sample stage was translated. According to the preferred embodiments of the present invention, by taking a holographic approach, entire areas can be written at once. This is analogous to parallel processing while the previous approaches were serial in nature.
Additionally, in various preferred embodiments, ultrafast holography is used to initiate H-TPIP of a reactive monomer mixed with liquid crystal. This results in a switchable micro or nano-grating structure having multiple applications.
The above and other embodiments, aspects, features and advantages will be further appreciated based upon the following description of the preferred embodiments of the invention.